[gdb/testsuite] Remove address from test names

I noticed an address in a test name:
...
PASS: gdb.base/eh_return.exp: gdb_breakpoint: \
  set breakpoint at *0x000000000040071b
...

Stabilize the test name by using "set breakpoint on address" instead.

Likewise in two other test-cases.

Tested on x86_64-linux.